In Hero of Ages, Sazed's notes explain that before the rise of the Lord Ruler, mistings existed, but not mistborns. The first mistborns were created by the Lord Ruler by ingesting pieces of Preservation's body. Somehow, this power was transferable through lineage (genetics are not necessarily involved) and, as a thousand years passed, its strength decreased.
The natural state of allomancers seems to be that of the simple misting, having only one power. This seems to be how Preservation wanted it. The Lord Ruler changed that, however, and the noble families, having more of Preservation's power through lineage, have better chances to produce mistings, the natural occurence of allomancy. Sometimes, though, because of this meddling, Preservation's power is more concentrated in an individual, giving them the power of a mistborn.
As for Spook, he always felt he was useless in the crew, as his powers as a Tineye were very passive. He yearned to be able to fight with the others, yet lacked the power to do so even though he could sense things the others could not. I always interpreted his musings to mean that he felt complete because he could now act on the information his senses provided and, given his nature as a tin savant, pewter gave him the physical resolve to endure the negative effects of his constant tin flaring.